Catalyst Design And Mechanism Study For The Efficient Production Of Bio-based Polycarbonate From Isosorbide

Polycarbonate(PC)is an engineering plastic widely used in optical materials,electronic devices,mechanical equipments and medical instruments.However,the synthesis of the conventional PC mainly relies on bisphenol A(BPA),a petroleum-based monomer with biological chronic toxicity.Currently,the poly(isosorbide carbonate)(PIC)synthesized by isosorbide derived from biomass is considered to be the most potential substitute for traditional PC,because of the excellent thermal properties and transparency of PIC.However,the synthesis of high-molecular-weight PIC is the main challenge due to the activity restriction of the intermolecular hydrogen bonds in isosorbide.Accordingly,two green catalytic systems including dication ionic liquids(DILs)and deep eutectic solvents(DESs)were designed and used to catalyze the synthesis of high-molecular-weight PIC with diphenyl carbonate and isosorbide as raw materials via the melt transesterification.Meanwhile,the active mechanism of the catalysts was investigated.The detailed research contents are as follows:(1)A series of high-activity DILs were designed to balance the reactivity of the endo-hydroxyl(endo-OH)and exo-hydroxyl(exo-OH)groups of isosorbide to synthesize PIC with high molecular weight.The results showed that when the trace amounts of bis-(3-methyl-l-imidazole)-ethylene dibromide was used,the weight-average molecular weight(Mw)of PIC reached 98700 g/mol.It could be concluded from the results of the experiment and the density functional theory stimulation that the high catalytic activity of DILs was attributed to the strong electrostatic interaction between the cation and the substrate and the effective balance of the reactivity of the endo-OH and the exo-OH.Furthermore,the influence of the molecular weight and chain configuration of PIC on its thermal performance was systematically analyzed.The results indicated that its thermal decomposition temperature could be effectively adjusted by reducing the content of terminal hydroxyl groups in the PIC molecular chain.The glass transition temperature of PIC was improved by increasing the endo-endo(a1)structure in the repeating unit.Finally,combining characterization results of Proton Nuclear Magnetic Resonance Spectroscopy(1H NMR)and Fourier Infrared Spectroscopy(FT-IR),a reaction mechanism based on multi-site activation of cations and synergistic effect of anions was proposed.(2)1-Hydroxyethyl-3-methylimidazole chloride([EmimOH]Cl)was selected as the hydrogen bond acceptor(HBA),a series of low-cost DESs with different pH were successfully prepared by adjusting the acidity and basicity of hydrogen bond donor(HBD)and used in the synthesis of PIC.The results showed that the molecular weight of the PIC prepared from the near-neutral[EmimOH]Cl-2EG was the highest with the MW of 102000 g/mol.The results of the transesterification kinetics with different catalysts indicated that the transesterification rate was significantly promoted as the basicity of DESs increased,but strong alkalinity inhibited the MW of PIC.Therefore,the degradation of PIC was investigated to reveal the mechanism of the basicity on molecular weight of PIC.The experimental results illustrated that the strong alkalinity of DESs would cause the depolymerization of the macromolecular chains and ultimately limit the molecular weight of the product,proving that a near-neutral catalyst was more conducive to the Mw of PIC.Combining 1H NMR,FT-IR characterization and kinetic experimental results,a reaction mechanism based on the synergistic catalysis of HBD and HBA was proposed.